This invention relates to real time error processing in automatic control systems for the operation of devices and equipment generally. More particularly this invention relates to a system for modifying the switching point in on-off operation of the heater in an incubation system for samples in automatic chemical analysis instruments.
In the control of equipment such as motors and heaters which have high power demand, discrete level control, typically on-off control systems, is generally more predictable and less expensive than a proportional control system for attempted continuous levels of operation. However, even when on-off switching is performed accurately, power transmission and heat transfer can require a time delay resulting in "overshoot and undershoot" relative to the desired value of the controlled variable.
In modern laboratory analysis, for example, analysis of blood serums, large numbers of serum samples must be processed typically for enzyme reaction analysis under controlled temperature. High speed, automatic instruments such as the instruments described in U.S. Pat. No. 4,234,538 have been developed to perform such reaction measurements. This patent describes instruments for photometric measurement of the reaction of large numbers of samples.
Accurate determination of such enzyme reaction measurements requires that the measurements be made at known temperatures; when repeated measurements over time are to be made, a practical instrument must be capable of maintaining the reaction vessels and temperature-sensitive reactions within continuous tolerance in order to eliminate difficult temperature correction, particularly in the reaction and measurement of large number of samples. Thus, controlling overshoot and undershoot of the reaction temperature within acceptable tolerance is critically important to a reliable instrument.
In reissued U.S. Pat. No. Re. 29,880, Duff describes an instrument for photometric measurement in which the sample vessels can be mounted in a relatively large, easily controlled heating chamber in order to maintain desired temperature. Such direct heating systems are generally inadequate for complex instruments.
U.S. Pat. No. 3,916,152 describes a temperature control system for an analyzer instrument in which the samples are heated and the rate of temperature increase is determined. A rate limiter circuit compares the rate of temperature increase to a predetermined value, which is pre-selected by trial and error, and when the rate of increase exceeds the predetermined value, the rate limiter circuit signals a trigger to switch off the heater. When the temperature of the sample cuvette falls below the desired predetermined temperature value, the trigger circuit is separately signaled to switch on the heater. Thus, the rate limiter circuit functions only to turn off the heater when the rate limiter signal (the rate of temperature increase) is of sufficient, predetermined magnitude to pass a hold-off gate and to deactivate the pulses from the heater trigger. Consequently, the switching point for the heater is fixed by entirely predetermined limits.